I preferred the HD 650 as well.
Better bass extension, smoother upper mids, more organic sounding.
I always found the 600 thinner, brighter, and a touch grainy in the upper mids/highs compared to the 650.
The subbass was also alittle weaker on the 600's.
I also actually felt the 650's had a tad more resolution and a slightly wider soundstage.
On the other hand, the 600's maybe had a touch more treble extension, or maybe it was more treble energy?.
But still, in all honesty, they really don't sound that much different, unlike the Hifiman HE-500 vs HE-6, or Beyer DT880 vs DT990, for example.
As always though, YMMV.

solitary man said:

  Uh, yeah, let's not hand the blow torch to the chimp, jump the shark and screw the pooch. I'm from the school of if it isn't broken don't fix it. Any product should work right out of the box or not, just that simple.

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????
 
 
I never said do mods on the HD600, I meant that if you were up for some DIY you can get the bottlehead crack amp as it comes in a kit that you put together yourself. It is one of the best amps for the HD600 and it doesn't break the bank.

solitary man said:

What the heck is "euphonic"?

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From somewhere else on this site:
Euphonic - An appealing form of distortion that generally enhances perceived fidelity, often ascribed to the harmonic elaborations of some valve amps. Coloration/euphonics are what the public generally want. It is somewhere in most media players and amplifiers. How a headphone is able to achieve euphony I don't actually know. Supposedly one of the worlds finest headphones Sennheiser's electrostatic HE90 is said to be euphonic.

kramer5150 said:

  Lets not forget cosmetically one is a gun-metal, pewter metallic with black cable... the other is faux marble with thinner / colored cable connectors.
 
Since this thread seems to be going that direction, I prefer the warm groovy HD650.

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None of that stuff matters to me. Comfort and sound is where it's at where I am concerned.

harleyzk said:

 
????
 
 
I never said do mods on the HD600, I meant that if you were up for some DIY you can get the bottlehead crack amp as it comes in a kit that you put together yourself. It is one of the best amps for the HD600 and it doesn't break the bank.

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Sorry, I miss understood. I've been reading about mods everywhere and shaking my head, so I thought that's what we were talking about.
I can't solder, otherwise I'd look at that amp kit.

solitary man said:

   sound is where it's at where I am concerned.

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So, what kind of sound?  We can talk ourselves silly about preferences but it's all up to you.